Electron microscopes exhibiting improved imaging of specimen having chargeable bodies

ABSTRACT

Electron microscopes (e.g., scanning electron microscopes, mapping SEMs) are disclosed in which the amount of charging of the specimen is controlled to between a minimum amount needed to view an image and a maximum amount beyond which a viewable image cannot be obtained, and such that the image has low distortion and the specimen is not damaged. Multiple irradiation-electron beams, or multiple segments of a single irradiation-electron beam, are directed to a specimen surface. The irradiation beams (or segments) are decelerated by a retarding voltage applied by a cathode lens and are incident on the specimen surface. The respective current and incident energy of each irradiation beam (or segment thereof) are controlled independently to a predetermined relationship so as to impart predetermined amounts of charging to different insulator regions of the specimen.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of, and claims the benefit of,co-pending U.S. patent application Ser. No. 09/583,001, filed on May 26,2000. The entire '001 application is incorporated by reference into theinstant application.

FIELD

[0002] This disclosure pertains to electron microscopes and relatedelectron-optical systems with which it is possible to view a specimensurface in two dimensions.

BACKGROUND

[0003] A scanning electron microscope (SEM) generally is used forexamining the surface of a specimen, such as the product of a step in aprocess for manufacturing semiconductor integrated circuits, especiallyto ascertain the presence of surficial defects. In view of the fact thatan electron beam is an exemplary charged particle beam, investigationshave been made into the use of other charged particle beams (such as afocused ion beam) for similar applications.

[0004] Since principles generally applicable to an electron beam areapplicable to an ion beam, the discussion below is made in the contextof an electron-beam system. However, in view of the above, it will beunderstood that the invention is not limited to electron-beam systems.

[0005] In an SEM, as is known generally, an electron beam is irradiatedonto a point on the surface of the specimen being observed. Impingementof the electron beam on the specimen surface causes the surface to emitsecondary electrons. The secondary electrons are accelerated away fromthe surface, collected, and quantified by a suitable detector. To imagea region on the sample, the electron beam simply is scanned in twodimensions in a raster manner. Secondary electrons generated at eachirradiation point in the scan are collected and quantified. The datacollected by the detector are processed to form an image that isdisplayed on a screen (CRT) or the like.

[0006] A main disadvantage of conventional SEMs is the long period oftime required for obtaining an image of the surface being observed. Thetime is related to the need to scan a point-focused electron beamtwo-dimensionally over the observed surface. As a result, “mappingelectron microscopes” are being investigated for use, as a possiblealternative to SEMs, in examining semiconductor wafers and chips and inother applications in which high speed is required. This is because amapping electron microscope offers prospects of simultaneously viewingan entire region of the target surface in two dimensions. To such end, amapping electron microscope utilizes an electron-optical system (i.e., asystem comprising a 2-dimensional projection-electron lens) to directthe electron beam onto an area of the sample surface that is larger thana point. Unfortunately, various technical problems remain unresolvedwith mapping electron microscopes.

[0007] An important technical problem (that is not limited to mappingelectron microscopy) concerns electrostatic charging of the specimensurface that is being observed. Charging can occur at locations on thespecimen occupied by insulators or floating conductors. During charging,the irradiated area acquires a positive or negative electrostatic chargewhenever the number (quantity) of the electrons irradiating the specimenis not equal to the number (quantity) of electrons emitted from theirradiated surface as secondary electrons and the like. Whenevercharging occurs, the observed surface of the specimen is not in adesired equipotential condition; in fact, the localized potentialswithin the observed field can differ to such an extent (due to localizedaccumulations of electrostatic charges) that imaging of certain regionsis impossible.

[0008] In various types of scanning electron microscopes, includingmapping electron microscopes, low-energy electrons, especially secondaryelectrons and the like, are accelerated and magnified to highmagnification and projected by an electrostatic lens onto an imagingsurface (e.g., the surface of a detector). The energy band of suchelectrons that can be imaged is narrow due to defocusing (on-axischromatic aberrations). Also, energy uniformity across the entireimaging field is difficult to sustain. Serious problems can arise if thedistribution of electrical potential varies greatly over the specimensurface because the image in the vicinity of such variations isdistorted or cannot be formed at all, making accurate observationimpossible. In addition, the specimen itself may be damaged if itbecomes charged sufficiently greatly to cause an electrostatic dischargeor insulation breakdown.

[0009] The occurrence of charging is determined at least in part by the“secondary-electron production efficiency.” The secondary-electronproduction (SEP) efficiency is the current of produced secondaryelectrons divided by the beam current of charged particles in the beamirradiating the specimen. If the SEP efficiency is greater than unity(1), then the specimen acquires a positive electrostatic charge; if theSEP efficiency is less than unity, then the specimen acquires a negativeelectrostatic charge. Hence, to avoid the problems summarized above, itwould be advantageous if specimen irradiation could be performed(especially with respect to insulators and floating conductors) in amanner by which the SEP efficiency is maintained as close to unity aspossible.

[0010] However, a typical specimen (especially a patterned semiconductorwafer or chip) typically includes multiple types of insulators andfloating conductors each having a different respective SEP efficiency.With such specimens, it is conventionally extremely difficult to observethe specimen by scanning or mapping electron microscopy without causingunacceptable levels of localized charging. Many specimens simply cannotbe imaged at all without intentionally charging them at least to acertain extent (e.g., to obtain a potential-contrast image). In suchinstances, it is difficult or impossible to control the extent oflocalized or general charging of the specimen.

SUMMARY

[0011] The shortcomings of the prior art as summarized above are solvedby electron microscopes as disclosed herein in which the degree oflocalized charging of the specimen is controlled, especially withrespect to insulators and floating conductors. Hence, the charging ismaintained between a minimum needed for producing a viewable image and amaximum beyond which a viewable image is not obtainable withsufficiently low distortion or without damaging the specimen.

[0012] A representative embodiment of an electron microscope comprisesan irradiation-optical system situated and configured to irradiate atwo-dimensional region of a surface of a specimen with a beam of chargedparticles produced by a charged-particle source. The irradiation of theregion causes emission of imaging electrons from the irradiated region.The embodiment also includes an imaging-electron detector having adetection surface. The embodiment also comprises animaging-electron-optical system situated and configured to direct theimaging electrons onto the detection surface, wherein theirradiation-optical system controls the beam of charged particles insuch a manner that changes in potential in the irradiated region due tocharging by the charged particles are within a range in which an imagecan be obtained.

[0013] The imaging optical system further can be configured to irradiatemultiple regions on the specimen surface such that the regions acquirerespective changes of surface potential (U_(s)) that are greater thanrespective minimum changes of surface potential (U_(min)) needed toproduce a viewable image and respective maximum changes of surfacepotential (U_(max)) beyond which a viewable image cannot be obtained. AWien filter can be included to direct the beam of charged particles fromthe irradiation-optical system to the specimen surface. Using a Wienfilter allows for perpendicular irradiation of the specimen, whichfacilitates uniform irradiation of the specimen, compared to angledirradiation. Koehler illumination conditions can be created by placingan aperture between the Wien filter and the specimen, and aligning thefocal point of a cathode lens (located between the aperture and thespecimen) with the aperture position.

[0014] The detector can include an imaging-electron converter and aphotoelectric converter, such as a charged-coupled device (CCD). Thedetector receives the imaging electrons and converts them to acorresponding electrical signal.

[0015] Desirably, irradiation of the region of the specimen surface isperformed under uniform irradiation conditions. This produces a clearimage without lightening or darkening of the image based on localizedcharging or irradiation irregularities within the region.

[0016] Also provided are methods for performing electron microscopy of asurface of a specimen. An embodiment of such a method comprises placingthe specimen relative to an irradiation-optical system and irradiating atwo-dimensional region of a surface of the specimen with a beam ofcharged particles that have propagated from a source through theirradiation-optical system. The irradiation is performed so as to causeemission of imaging electrons from the irradiated region on thespecimen. The imaging electrons are directed to a detection surface ofan imaging-electron detector. Operation of the irradiation-opticalsystem is controlled so as to control the beam of charged particles in amannner such that changes in potential in the irradiated region due tocharging by the charged particles in the region of the specimen surfaceare within a range in which an image is obtained.

[0017] The foregoing and additional features and advantages of theinvention will be more readily apparent from the following detaileddescription, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic elevational section showing exemplaryinsulator bodies in a specimen irradiated with a single electron beamfor mapping electron microscopy.

[0019] FIGS. 2(a)-2(b) are respective graphs of representativerelationships between incident energy (of an electron beam) andsecondary-electron production efficiency of respective insulator bodiessuch as shown in FIG. 1.

[0020]FIG. 3 is a schematic elevational section showing exemplaryinsulator bodies in a specimen irradiated with multiple electron beamsfor mapping electron microscopy.

[0021] FIGS. 4(a)-4(b) are respective graphs of representativerelationships between incident energy (of an electron beam) andsecondary-electron production efficiency of respective insulator bodiessuch as shown in FIG. 3.

[0022]FIG. 5 is a schematic optical diagram of a mapping electronmicroscope according to a first representative embodiment.

[0023]FIG. 6 is a schematic optical diagram of a mapping electronmicroscope according to a second representative embodiment.

[0024]FIG. 7 is a schematic optical diagram of a mapping electronmicroscope according to a third representative embodiment.

DETAILED DESCRIPTION

[0025] General Considerations

[0026] As noted above, a typical specimen for electron microscopyincludes multiple individual insulator bodies and conductors. Therespective change of surface potential (U_(s)) of each such body orregion desirably is controlled so that the potential is between aminimum change in surface potential (U_(min)) needed to produce aviewable image and a maximum change in surface potential (U_(max))beyond which a viewable image cannot be obtained (due, for example, tothe image having excess distortion or the specimen being damaged byelectrostatic discharge). Also, irradiation of the specimen desirably isperformed under uniform illumination conditions within the imaging fieldto facilitate obtaining a clear image without localized lightening ordarkening from respective localized charging or irradiationirregularities within the field.

[0027] As used herein, “imaging electrons” are electrons emitted fromthe specimen or other surface due to irradiation by a charged particlebeam. Imaging electrons include, for example, reflected electrons,secondary electrons, and backscattered electrons.

[0028] The changes of surface potentials noted above desirably have thefollowing relationship for optimal imaging:

U _(min) <U _(s) <U _(max)  (1)

[0029] The efficiency with which imaging electrons are emitted from anirradiated region of the specimen is a function of the energy of theirradiating electrons (or other charged particles), the substance andstructure of the specimen, the imaging environment, and other factors.

[0030] Most substances used in semiconductor-fabrication processes havea secondary-electron production (SEP) efficiency greater than unity (1)in irradiating-electron fields of 100 eV to 1 KeV, but less than unityin fields outside this range. Also, whenever a floating conductor orinsulator is irradiated for a period of time, the conductor or insulatoraccumulates charge and exhibits a corresponding change in potential overtime. If the specimen has only one type of floating conductor orinsulator, and if the specimen is uniform, then the specimen can beirradiated readily with a single beam having an energy that will yield aSEP efficiency of unity. However, if the specimen comprises multipletypes of substances (which is the case with most semiconductor wafersand chips), then the irradiation energy that will yield a SEP efficiencyof unity typically will differ for each of the constituent substances.Under such conditions, optimal imaging of all regions of the specimensurface usually cannot be performed using a single beam having a singleenergy level. Either multiple beams are required (each having arespective energy level), or a single beam with multiple energy levelsis required.

[0031] For example, consider a specimen having an elevational sectionalprofile as shown in FIG. 1, in which regions 2, 3 are respectiveinsulator bodies A and B in a silicon substrate 1. The substrate 1 iselectrically conductive whereas the insulator bodies A, B are not. Thetop surface of the specimen is irradiated (arrows 4) by incidentelectrons within an illumination field with the intention of producingan image of the specimen surface. Because the specimen in this examplehas a planarized top surface (as achieved by a suitable technique suchas CMP), image contrast generally will be too low for obtaining a goodimage by optical microscopy or even by edge-emphasized SEM microscopy.

[0032] Upon illuminating the FIG. 1 specimen with electrons 4 having anincident energy of V₁, localized charging occurs that tends to changethe incident energy of electrons on regions undergoing localizedcharging. To the extent that there is no leakage current, the incidentenergy on the insulator bodies A, B shifts from the initial value of V₁to respective levels “a” and “b” as shown in FIGS. 2(a)-2(b),respectively. The levels “a” and “b” correspond to respective SEPefficiencies of the respective insulator bodies A and B at irradiationequilibrium. As a result, the change of (shift in) the chargingpotential of the insulator bodies A (U_(s/A)) and the change of (shiftin) the charging potential of the insulator body B (U_(s/B)) increasesby (a−V₁) and (b−V₁), respectively.

[0033] The shifts in charging potentials U_(s/A) and U_(s/B)simultaneously may fulfill the following two inequalities:

U _(min) <U _(s/A) <U _(max)  (2)

U _(min) <U _(s) /B<U _(max)  (3)

[0034] (Note that, above, U_(min) and U_(max) are the same for each bodyA and B.) However, there generally are many instances in which theseconditions cannot be achieved, even if the respective levels of V₁ arechanged in FIGS. 2(a) and 2(b).

[0035] Now, assume that the specimen is irradiated simultaneously withelectrons 4 having an incident energy of V₁ and electrons 5 having anincident energy of V₂, as shown in FIG. 3. V₁ and V₂ are selected so asto be situated on opposite sides of the equilibrium points a and b ofthe respective insulator bodies A and B, as shown in FIGS. 4(a) and4(b), respectively. The respective shifts in charging potential U_(s/A),U_(s/B) of the insulator bodies A and B, irradiated with the electronsof two different energies V₁, V₂, are found as follows.

[0036] The SEP efficiency functions for the insulator bodies A and B, asfunctions of the irradiation electron energy V, are denoted FA(V) andFB(V), respectively. The respective irradiation-electron beam currentsat the specimen surface of the beams having respective incident energiesof V₁ and V₂ are I₁ and I₂, respectively. The respectivesecondary-electron beam currents emitted from the surfaces of therespective insulators A and B when irradiated at the respective incidentenergies V₁ and V₂ are expressed as:

from A: I ₁ −FA(V ₁)+I ₂ ·FA(V ₂)  (4)

from B: I ₁ ·FB(V ₁)+I ₂ ·FB(V ₂)  (5)

[0037] As can be seen, the respective sums indicated in Expressions (4)and (5) generally are not the same as the sum of the incidentirradiation-beam currents:

I ₁ +I ₂  (6)

[0038] Rather, in Expressions (4) and (5), each of the I₁ and I₂ termsis factored by the respective SEP efficiency (FA) function. As a result,charging of the respective insulator bodies occurs until equilibrium isreached. At equilibrium, the incident energy V₁ is shifted to V₁+U bythe charge-up potential U, and the respective shifts in surfacepotential U_(s/A), U_(s/B) reach the following respective steady-stateconditions:

for A: I ₁ +I ₂ =I ₁ ·FA(V ₁ +U _(s/A))+I ₂ ·FA(V ₂ +U _(s/A))  (7)

for B: I ₁ +I ₂ =I ₁ ·FB(V ₁ +U _(s/B))+I ₂ ·FB(V ₂ +U _(s/B))  (8)

[0039] If the variable a is denoted as follows:

α=I ₁/(I ₁ +I ₂)  (9)

[0040] then Equations (7) and (8) can be written respectively as:

for A: 1=α·FA(V ₁ +U _(s/A))+(1−α)·FA(V ₂ +U _(s/A))  (10)

for B: 1=α−FB(V ₁ +U _(s/B))+(1−α)·FB(V ₂ +U _(s/B))  (11)

[0041] If U_(s/A) and U_(s/B) and one of α, V₁, and V₂ are establishedat specific values that fulfill Expressions (2) and (3), then theremaining two variables can be determined so that both Equations (10)and (11) are satisfied, thereby allowing the specimen to be imaged withgood results. Moreover, by changing the total irradiation-currentdensity, illumination can be accomplished under even better irradiationconditions because the image is brightened.

[0042] As can be ascertained from the foregoing, if α, V₁, and V₂ inEquations (10) and (11) are all found as variables, then the respectivevalues can be applied to up to three types of insulators. (For example,for a third insulator FC(V) experiencing a shift in charging potentialU_(s/C), 1=α·FC(V₁+U_(s/C))+(1−α)·FC(V₂+U_(s/C)).) Since the number ofnew variables increases by two (i.e., an additional incident-energy (V)term and an additional beam-current (I) term is added) for eachadditional beam supplying another level of irradiation electron energy,the number of types of insulators to which these principles can beapplied also increases by two under such conditions. (I.e., each beamhas two variables V and I. Simultaneous equations involving thesevariables have two solutions.)

[0043] Hence, the irradiation-optical system can be configured such thatthe specimen being observed is irradiated simultaneously by illuminationfrom multiple electron sources each having a respective current (I) andincident energy (V) that are controlled independently, as describedabove. The respective currents and incident energies can be establishedto maintain the changes in the surface potential due to charging withinrespective target values for each insulator or floating conductor. Thisallows the surface potential (U_(s)) due to charging to be controlledfor each insulator and/or floating conductor so that the surfacepotential is between a minimum amount (U_(min)) needed to view an imageand a maximum amount (U_(max)) beyond which a viewing image cannot beobtained with low distortion and/or without damaging the specimenitself.

[0044] The invention is further described below in the context ofrepresentative embodiments that are not intended to be limiting in anyway.

[0045] Representative Embodiment 1

[0046] This embodiment, depicted in FIG. 5, comprises irradiation-beamcolumns 11, 12, a Wien filter (E×B) 13, a cathode lens 14, aprojection-optical system 16, and a detection surface 17 (e.g., surfaceof a suitable detector of secondary electrons). The specimen surface isdenoted by the numeral 15.

[0047] The irradiation columns 11, 12 accelerate electrons fromrespective electron sources 11S, 12S and form the electrons intorespective beams of predetermined respective transverse profile andarea. The irradiation column 11 is situated at a respective angle θ₁ tothe optical axis AX, and the irradiation column 12 is situated at arespective angle θ₂ to the optical axis AX. The respective irradiationbeams propagate to the optical center of the Wien filter 13. Eachirradiation beam is deflected by the Wien filter 13 to propagate alongthe axis AX toward the specimen surface 15. Hence, the irradiation beamsare incident perpendicularly to the specimen surface 15. The irradiatingelectrons in the irradiating beams are decelerated by a retardingvoltage applied by the cathode lens 14 and are incident onto apredetermined area of the specimen surface 15.

[0048] An aperture (not shown) desirably is situated between the Wienfilter 13 and the cathode lens 14. By locating the aperture at the focalpoint of the cathode lens 14, the aperture serves to Koehler-irradiatethe specimen surface 15.

[0049] In the following discussion, the irradiation beams entering theWien filter 13 from the irradiation columns 11, 12 have respectiveenergies of V₁₁ and V₁₂, and respective incident energies V₁ and V₂. Thepotential energy imparted to the respective irradiation beams by theretarding voltage imposed by the cathode lens 14 is denoted V_(ret). Thepotential energy V_(ret) can be positive or negative relative to thespecimen surface 15, but normally is positive. The energies V₁₁ and V₁₂are expressed as, respectively:

V ₁₁ =V ₁ +V _(ret)  (12)

V ₁₂ =V ₂ +V _(ret)  (13)

[0050] The respective beam-current values of the irradiation beams fromthe respective columns 11, 12 are denoted I₁ and I₂, which are relatedto V₁, V₂, and α (=I₁/(I₁+I₂)) as set forth in Equations (10) and (11).

[0051] The respective deflection angles θ₁ and θ₂ of the irradiationbeams are established simultaneously according to the following:

L=(sin θ₁ /eB)(2m)^(1/2) V ₁₁/[(V ₁₁)^(1/2)+(V _(ret))^(1/2)]  (14)

L=(sin θ₂ /eB)(2m)^(1/2) V ₁₂/[(V ₁₂)^(1/2)+(V _(ret))^(1/2)]  (15)

[0052] within a magnetic field B that fulfills Wien conditions withrespect to the secondary electrons accelerated at a retarding voltage ofV_(ret), wherein “L” is the nominal thickness of the Wien filter 13, “e”is the absolute value of the charge of an electron, and “m” is the massof an electron. At known respective energies V₁₁, V₁₂, the deflectionangles θ₁, θ₂ can be set to any value that satisfies the relation:

sin θ₁/sin θ₂ =V ₁₂[(V ₁₁)^(1/2)+(V _(ret))^(1/2) ]/{V ₁₁[(V₁₂)^(1/2)+(V _(ret))^(1/2]})  (16)

[0053] If the deflection angles θ₁, θ₂ are set accordingly, then theincident energies can be selected in satisfaction of Equation (16). Itis possible to vary the respective irradiation-current densitiesrandomly by changing the parameters of the respective electron sources11S, 12S and irradiation columns 11, 12.

[0054] Representative Embodiment 2

[0055] This embodiment is shown in FIG. 6, and includes a second Wienfilter 18. All other components in this embodiment are similar tocorresponding components in the FIG. 5 embodiment and have the samerespective reference numerals. Irradiation electrons from theirradiation column 11 and irradiation electrons from the irradiationcolumn 12 enter the separate Wien filters 13, 18, respectively.Consequently, in this embodiment, the magnetic fields B in Equations(14) and (15) are independent. This configuration makes it possible tochange the electron energies of the various irradiation systemsindependently. The respective irradiation-current densities can bevaried randomly by changing the parameters of the respective electronguns and irradiation columns.

[0056] Representative Embodiment 3

[0057] This embodiment is shown in FIG. 7. Irradiation electrons fromthe irradiation column 11 and irradiation electrons from the irradiationcolumn 12 are irradiated at respective angles from the optical axis AXof the imaging system. Consequently, the respective electron energies ofthe various irradiation systems can be changed independently. Therespective irradiation-current densities can be varied independently bychanging the parameters of the respective electron guns and of therespective irradiation columns. However, with this embodiment, uniformillumination over the entire field is more difficult than with aperpendicular illumination scheme such as those of FIGS. 5 and 6.

[0058] In each of the representative embodiments described above, onlytwo irradiation columns 11, 12 are provided. However, greater numbers ofirradiation columns can be utilized. Increasing the number ofirradiation columns makes it possible (especially when there arenumerous types of insulators in or on the specimen surface 15) to impartchanges in surface potential for each insulator body by appropriatelycharging them up.

[0059] In addition, although not shown, an effect similar to parallelillumination from the multiple irradiation columns 11, 12 can beobtained using only one irradiation column that produces a beam of whichthe beam current and incident energy are changed periodically in arepeating manner in serial time segments. This scheme can be utilizedbecause charging is a phenomenon that occurs overlappingly in time andspace.

[0060] Whereas electron beams are utilized as the irradiating beams ineach of the representative embodiments described above, it will beunderstood that irradiation can be performed with equal facility usinganother type of charged particle beam such as an ion beam.

[0061] Whereas the invention has been described in connection withmultiple representative embodiments, it will be understood that theinvention is not limited to those embodiments. On the contrary, theinvention is intended to encompass all modifications, alternatives, andequivalents as may be included within the spirit and scope of theinvention, as defined by the appended claims.

What is claimed is:
 1. An electron microscope, comprising: anirradiation-optical system situated and configured to irradiate atwo-dimensional region of a surface of a specimen with charged particlesproduced by a charged-particle source to cause emission of imagingelectrons from the region on the specimen; an imaging-electron detectorhaving a detection surface; and an imaging electron-optical systemsituated and configured to direct the imaging electrons onto thedetection surface, wherein the irradiation-optical system controls thecharged particles such that changes in potential due to charging by thecharged particles in the region of the specimen surface are within arange in which an image can be obtained.
 2. The electron microscope ofclaim 1, wherein the imaging electron-optical system is configured suchthat each of multiple regions on the specimen surface is irradiated soas to acquire a respective change of surface potential (U_(s)) that isgreater than a respective minimum change of surface potential (U_(min))needed to produce a viewable image and a respective maximum change ofsurface potential (U_(max)) beyond which a viewable image cannot beobtained.
 3. The electron microscope of claim 1, further comprising aWien filter situated and configured to direct the beam of chargedparticles from the irradiation-optical system to the specimen surface.4. The electron microscope of claim 1, further comprising a cathode lenssituated between the Wien filter and the specimen, the cathode lensbeing configured to decelerate the beam by applying a retarding voltageto the beam.
 5. The electron microscope of claim 1, wherein the imagingelectron-optical system is configured to change beam current andincident energy of the charged particles in the beam in a repeatingmanner in serial time segments.
 6. A method for performing electronmicroscopy of a surface of a specimen, the method comprising: placingthe specimen relative to an irradiation-optical system; irradiating atwo-dimensional region of a surface of the specimen by charged particlesthat have propagated from a source through the irradiation-opticalsystem, such that the irradiation of the region causes emission ofimaging electrons from the region on the specimen; directing the imagingelectrons through an imaging electron-optical system to a detectionsurface of an imaging-electron detector; and controlling operation ofthe irradiation-optical system so as to control the charged particles ina manner such that changes in potential due to charging by the chargedparticles in the region of the specimen surface are within a range inwhich an image is obtained.
 7. The method of claim 6, wherein each ofmultiple regions on the specimen surface is irradiated with theirradiation beam so as to exhibit a respective change of surfacepotential (U_(s)) that is greater than a respective minimum change ofsurface potential (U_(min)) needed to produce a viewable image and arespective maximum change of surface potential (U_(max)) beyond which aviewable image cannot be obtained.
 8. The method of claim 6, whereinoperation of the imaging electron-optical system is controlled so as tochange beam current and incident energy of the charged particles in thebeam in a repeating manner in serial time segments.